iommu/omap: Check for failure of a call to omap_iommu_dump_ctx

commit dee9d154 upstream.

It is possible for the call to omap_iommu_dump_ctx to return
a negative error number, so check for the failure and return
the error number rather than pass the negative value to
simple_read_from_buffer.

Fixes: 14e0e679

 ("OMAP: iommu: add initial debugfs support")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20200714192211.744776-1-colin.king@canonical.com


Addresses-Coverity: ("Improper use of negative value")
Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>